Electroluminescent devices based on polymeric thin films

Abstract

This thesis is concerned with the preparation of organic light-emitting diodes (LEDs) by using different thin film technologies: the Langmuir-Blodgett (LB) technique; spin-coating and thermal evaporation. The π-conjugated polymer, poly(2-methoxy-5-(5'-ethylhexyloxy)-p-phenylenevinylene) (MEH-PPV), was used as the emissive layer and was deposited onto patterned indium-tin oxide (ITO) glass using the LB technique or spin-coating. Y-type LB films of MEH-PPV were deposited at a surface pressure of 17 mN m(^-1) with a transfer ratio of 0.95 ± 0.03. Many efforts were made to improve the LB film device performance parameters, such as external quantum efficiency and operating lifetime, by inserting an electron transporting or insulating layer between the emissive layer and top cathode. Annealing the LB films was found to result in an improved operating lifetime. LEDs based on spun films possessed higher external quantum efficiencies than devices made from LB films. The more ordered LB films had a higher probability of intra- and intermolecular interactions and formed more excimer states within the structure. This led to a lower quantum efficiency compared to devices incorporating spun films. The operating lifetime of the LEDs was highly dependent on the morphology of the film surface. A smoother film surface is required for a longer device operating lifetime. A new electron transporting material, 2,5-bis[2-(4-tert-butylphenyl)-1,3,4-oxadiazol-5-yl]pyridine (PDPyDP), was deposited on top of the MEH-PPV spun film. Despite a high external quantum efficiency of 0.7 %, this device suffered from dark regions in the electroluminescence output resulting in degradation of the device. The dark area formation was attributed to delamination of the aluminium electrode from the PDPyDP layer, which was lessened by: (a) annealing the degraded devices; (b) evaporating a thicker aluminium layer at a high rate and (c) inserting a buffer layer (Alq(_3)) between the PDPyDP and the Al top electrode.